1. Field of the invention
The present invention relates to a micro-structured fiber having an array of micrometer-sized air holes, and more particularly, to a micro-structured fiber having a plurality of air holes with different diameters.
2. Description of the Related Art
Referring to FIG. 1, a cross-sectional view of a conventional micro-structured fiber is shown. The micro-structured fiber 10 is a so-called photonic crystal fiber (PCF). As shown in FIG. 1, the micro-structured fiber 10 comprises a core region 11 and a cladding region 12, wherein the material of the core region 11 is the same as that of the cladding region 12. The cladding region 12 has a plurality of air holes 13 that are approximately circular in shape and are regularly arranged on a plurality of hexagonal rings 14. The innermost ring 14 of the fiber 10 defines the core region 11. All of the air holes 133 have the same diameter d, and the pitches (distances) Λ between adjacent air holes, for example, between air holes 131 and 132, are all equal.
In order to achieve the chromatic dispersion related application, such as flattened dispersion, dispersion compensation or shifted dispersion, the value of the diameter d of the air holes 13 and the pitch Λ between the air holes 13 should be adjusted. In general, the larger ratio of the diameter d to the pitch Λ (d/Λ) of the air holes 13 represents that the light communication band has a larger normal dispersion value to realize dispersion compensation, whereas the smaller ratio of the diameter d to the pitch Λ (d/Λ) of the air holes 13, such as 0.25, realizes the flattened dispersion. In most cases, the pitch Λ is usually about 2.6 μm, and the diameter d is usually about 0.624 μm.
U.S. Pat. Nos. 6,636,677 and 6,718,105 disclose the fibers that achieve a better dispersion compensation by combining the technique of controlling the distribution of the refractive index around the core region and the structure of the micrometer-sized air holes. Additionally, U.S. Pat. Nos. 6,571,045 and 6,445,862 also achieve a better dispersion compensation caused by the effect of different distribution of the refractive index around the core region by controlling the air holes to have a uniform structure period.
Referring to FIG. 2, a relationship between the chromatic dispersion value and the wavelength of a conventional micro-structured fiber of FIG. 1 is shown. As shown in FIG. 2, the conventional micro-structured fiber 10 has a dispersion of ±0.5 ps/km/nm from 1350 nm to 1750 nm wavelength, is which has a bandwidth of 400 nm. It is known that the photonic crystal fiber (PCF) with flattened dispersion has a ratio (d/Λ) of about 0.25. There are two ways to make the ratio (d/Λ) become smaller; one is to reduce the diameter d and the other one is to enlarge the pitch Λ. However, such two ways both will increase the confinement loss, and more than twenty rings of air holes are required in the cladding region 12 to significantly reduce the confinement loss to the standard of conventional fiber. Therefore, such design raises the difficulty in manufacture and the manufacturing cost (see “A. Ferrando, E. Silvestre, J. J. Miret, and P. Andres, “Nearly zero ultraflattened dispersion in photonic crystal fibers,” Opt. Lett., vol. 25, pp. 790–792, 2000” and “W. H. Reeves, J. C. Knight, and P. St. J. Russell, “Demonstration of ultra-flattened dispersion in photonic crystal fibers,” Opt. Expr., vol. 10, pp. 609–613, 2002”).
Recently, a second prior art of a design of photonic crystal fiber (PCF) with four or five rings of different air-hole diameters for each ring was proposed for achieving ultralow ultra-flattened dispersion (see “K. Saitoh and M. Koshiba, “Chromatic dispersion control in photonic crystal fibers: Application to ultra-flattened dispersion,” Opt. Expr., vol. 11, pp. 843–852, 2003”, “K. Saitoh and M. Koshiba, “Unique Dispersion Properties of Photonic Crystal Fibers,” ICICS-PCM 2003, pp. 171–175, December 2003” and “F. Poli, A. Cucinotta, S. Selleri, and A. H. Bouk, “Tailoring of Flattened Dispersion in Highly nonlinear Photonic Crystal Fibers,” IEEE Photon. Technol. Lett., vol. 16, pp. 1065–1067, April 2004”). This design significantly reduces the ring number of the air holes, but the design procedure becomes complicated due to several geometrical parameters, five (four kinds of air-hole diameters and one pitch) for the four-ring case and six (five kinds of air-hole diameters and one pitch) for the five-ring case, which are needed to be simultaneously optimized to achieve the flattened dispersion behavior of the PCF.
Consequently, there is an existing need for a novel and improved broadband ultra-flattened dispersion micro-structured fiber to solve the above-mentioned problems.